Union entities also ponied up serious eleventh-hour cash, including the Laborers International Union of North America ($1 million on Nov. 5), International Brotherhood of Electrical Workers ($500,000) and the United Brotherhood of Carpenters and Joiners’s Working for Working Americans super PAC ($500,000).

In all, Priorities USA Action spent nearly $21 million between Oct. 18 and Nov. 26 while ending the period with $4.33 million cash on hand and no debt.

While most of the money it spent in late October and early November funded anti-Romney ads, the super PAC did spend six-figure amounts on research and media strategy services, its report indicates.

Such spending is remarkable considering Priorities USA Action began the year destitute, barely raising any money at all as Democrats, largely on principle, shunned supporting super PACs, which by law may raise and spend unlimited amounts of money to promote or attack candidates.

But Obama in February gave tacit approval to his funders backing Priorities USA Action, which is run by two of his former White House staffers, Bill Burton and Sean Sweeney.

From that point forward, cash slowly began to roll in, even if Priorities USA Action never competed dollar for dollar with its main Republican super PAC counterparts, pro-Romney Restore Our Future and Karl Rove-backed American Crossroads. In August, it had raised $10.13 million, and in September, it raised $15.2 million, adding $13 million more during the first 17 days of October, its federal reports show.